Engine air/fuel feedback control systems are known in which a feedback variable derived from an exhaust gas oxygen sensor trims fuel flow to the engine in an effort to maintain stoichiometric combustion. A two-state oxygen sensor is typically used in which the change in output state occurs at stoichiometry under ideal conditions. The system includes a three way catalytic converter which has a peak efficiency window at stoichiometry under ideal conditions. Optimal catalytic conversion of hydrocarbons, carbon monoxide, and nitrogen oxides occurs at the peak efficiency window.
The inventors herein have recognized numerous problems with the above approaches. For example, the transition in exhaust gas oxygen sensor output states may not occur at stoichiometry for all sensors or over the life of any particular sensor. Furthermore, the peak efficiency window may not occur at stoichiometry for all catalytic converters. Accordingly, engine air/fuel ratio may not occur at the converter's peak efficiency window, thus resulting in less than optimal conversion of engine exhaust.